This invention relates generally to an electrophotographic printing machine, and more particularly concerns a printing machine wherein the latent image recorded on the photoconductive member is modulated by a screen pattern having a regulatable contrast.
In a typical electrophotographic printing machine, a latent image is recorded on a photoconductive member and developed with toner particles. A sheet of support material is positioned closely adjacent to the latent image and arranged to have the particles transferred thereto. After the particles are transferred to the sheet of support material, they are permanently affixed thereto forming a copy of the original document. The latent image is recorded on the photoconductive member by projecting a light image of the original document onto the charged portion thereof. This dissipates the charge in accordance with the intensity of the light image to record the electrostatic latent image on the photoconductive member.
Multi-color electrophotographic printing is essentially the same as black and white printing. However, in multi-color printing, the foregoing is repeated a plurality of cycles, each cycle being for a discrete color. In this process, the light image is filtered to record an electrostatic latent image on the photoconductive member corresponding to a single color. A plurality of different single color light images are formed. Each single color electrostatic latent image is developed with toner particles complementary in color to the color of the filtered light image. The toner powder images are transferred to the sheet of support material in superimposed registration with one another. Heat is then applied to the multi-layered toner powder image permanently affixing it to the sheet of support material. This produces a permanent color copy of the original document.
Pictorial copies may be reproduced by employing a half-tone screen to form tone gradations. The screen produces tonal gradations by forming half-tone dots or lines of varying size. In the highlight regions, the half-tone pattern may comprise narrow lines or small dots. The lines increase in width or the dots in size throughout the intermediate shades until they merge together at the shadow end. This results in nearly complete whiteness at the highlight end and nearly solid blackness at the shadow end of the tonal scale.
Various patents such as U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010; 3,493,381; 3,776,633; and 3,809,555 teach the concept of screening. In general, the light image of the original document may be projected through the screen prior to exposing the charged portion of the photoconductive surface so as to produce a half-tone electrostatic latent image on the photoconductive surface. However, a system of this type requires high intensity lamps in order to provide sufficient light to illuminate the original document and have the reflected light rays pass through the screen onto the charged portion of the photoconductive member. This problem may be eliminated by projecting a light image of the screen pattern onto the charged portion of the photoconductive member prior to, or subsequent to the projection of the light image of the original document thereon. In this type of operation, the latent image of the screen pattern is recorded in superimposed registration with the latent image of the original document so as to record a modulated latent image on the photoconductive member. Hereinbefore, systems of this type employed a light source having a substantially constant illumination for irradiating the screen. However, it has been found that a constant light source does not permit variations of contrast nor does it readily facilitate changing the process response curve.
Accordingly, it is a primary object of the present invention to control the intensity of illumination irradiating the screen member so as to regulate contrast and adjust the process response curve.